Sons of Gondor, of Rohan. My brothers. I see in your eyes the same fear that would take the heart of me! A day may come, when the courage of men fails, when we forsake our friends and break all bonds of Fellowship, but it is not this day!

An hour of wolves and shattered shields when the age of men comes crashing down! But it is not this day!

This day we fight! By all that you hold dear on this good earth, I bid you, stand, men of the West!

I can handle them alright. For a xenos race, they were quite nice. Very polite and pleasant. Would never harm a soul.
And that's the thing; they meant well. Granted, they did commit horrific atrocities and spread the taint of Chaos to a mostly-unspoiled world, but they meant well. That's a damn shame, we could've done with some nicety around here.

Oh, there's niceness all over the place. It's just drowned in the amounts of death, misery and the like.
Being nice is simply reserved for the common citizens of the Imperium. You know, people who haven't decided/been forced to go off and face the horrific monstrosities that plague all humans across the Imperium.

As for the Tau, they're little better than any other race seeking to carve out their own empire. They're not inherently nice, just disciplined and self-contained. Their Water Caste diplomats might show niceness, but deny them and soon you'll have a Liberation Fleet bearing down on your planet.
As for mind control, that's false. All the Ethereals have to do is offer a better standard of life; exactly the same as any human citizen's previous life, with lower taxes or something with some xenotech thrown in to sweeten the deal. For the average citizen (Not the mindlessly devout/crazy/actually interesting ones) this is a fair price for betraying the Emperor, I am sorry to say.